The present invention relates to a method of fabricating a compound semiconductor thin-film solar cell having a p-type light absorbing layer with an n-type heterojunction buffer layer formed thereon.
FIG. 1 shows a basic structure of a thin-film solar cell produced from a general compound semiconductor, which comprises a SLG (soda lime glass) substrate 1 on which a back molybdenum (Mo) electrode layer (positive electrode) 2, a p-type light absorbing layer 5, an n-type heterojunction buffer layer 6 and a transparent electrode layer (negative electrode) 7 are subsequently formed in the described order.
In the thin-film compound semiconductor solar cell, the light absorbing layer 5 is produced in the form of a CIGS (Copper-Indium-Gallium-Selenium) thin film made of Cu (In+Ga) Se2 of I–III–VI2 group based on Cu, (In, Ga), Se, which possesses high power conversion efficiency exceeding 18%.
U.S. Pat. No. 4,611,091 discloses a method of forming a heterojunction buffer layer most suited to a light absorbing layer of CIS by chemically growing a thin film of CdS representing a compound semiconductor of II–VI group from a solution.
Japanese Laying-Open Patent Publication No. H-08-330614 describes a heterojunction buffer layer of ZnS which does not contain harmful metal such as cadmium and possesses high power conversion efficiency.
The above-described conventional methods of fabricating compound thin film solar cells involve such a common problem that, while the light absorbing layer is dipped in the solution, two processes of diffusing Zn or Cd element into the light absorbing layer and forming a ZnS or CdS film concurrently take place, which may easily cause variations in power conversion efficiency of the product from the crystallinity and surface conditions of the light absorbing layer.
The Chemical Bath Deposition (CBD) method may cause inclusion of impurities into a buffer layer to be formed on a light absorbing layer, which may have decreased quality.
The CBD method may also cause adhesion of ZnS to the inside wall of the bath and carriers transporting the substrates in the bath, thereby resulting in loss of ZnS and producing a large amount of waste solution.